Synthetic membranes have become important tools for separation and reaction engineering. An important driving force for developing advanced membranes is the need for more efficient technologies that produce and recycle pure fluids, particularly water. Membranes for pressure-driven filtration procedures, such as reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF), play a crucial role. However, in many applications, concentration polarization and membrane fouling limit membrane selectivity, capacity, and productivity. Filtration membranes prone to fouling have only limited usefulness, particularly for applications such as water treatment.
Numerous studies have shown that a variety of factors, not all of them independent, lead to membrane fouling. Fouling is the reduction of membrane performance due to deposition of matter on or in the selective barrier of the membrane. Fouling decreases membrane performance over time by reducing membrane selectivity (ability to separate components) and capacity (throughput). Fouling is often accelerated by concentration polarization. Concentration polarization occurs during tangential flow filtration and is due to convection of rejected species to the membrane surface by the permeate flow. Concentration polarization may be reduced by inducing mixing at the membrane surface to disrupt the boundary layer that forms.
Countermeasures against fouling include optimizing module design, inducing mixing during operation (cross-flow velocity, back pulsing), pretreatment of the feed, and surface modification of the membrane. The latter approach can largely reduce the adsorption or adhesion strength of colloidal or particular foulants, leading to less deposition and/or easier removal of fouling layers. The post-modification of established membranes has become an important approach. Examples of anti-fouling modification can be found in the literature, for instance, based on grafting thin layers of hydrophilic polymers to the surface of RO, NF or UF membranes. While such membranes typically have helpful “static” properties, more sophisticated strategies are based on so called responsive membranes. By using responsive membranes, fouling can be reduced or cleaning can be facilitated by surface properties (e.g., hydrophilicity, charge, modulus) that can be changed by external stimuli, e.g., pH value or temperature. However, with all such modified or even responsive membranes, concentration polarization cannot be influenced directly.
Although the use of membrane technology in industry is increasing, there is still a very real need to create fouling resistant membranes that are adaptable to a wide range of feeds. Accordingly, new polymeric liquid filtration membranes that have fouling resistant properties are needed by industry for numerous applications, including water filtration. There is also a need for filtration membranes that are adaptable to a wide range of feeds and that do not require the modification of bulk feeds to maintain their enhanced fouling resistance properties.